1. Field of the Invention
The present invention relates to an improvement of a thermo bulb for use with a thermostatic expansion valve.
2. Description of the Related Art
FIG. 1 schematically shows an overall arrangement of a refrigeration system provided with a thermostatic expansion valve 10. Thermostatic expansion valve 10 has a valve unit 14 located at an inlet of an evaporator 12 and a thermo bulb 16 located at an outlet of the evaporator 12 to sense the temperature of superheated refrigerant vapor. Thermostatic expansion valve 10 controls the extent of its valve opening in response to the difference (superheat) between the temperature of the superheated refrigerant vapor sensed at the outlet of the evaporator 12 by the thermo bulb 16 and the evaporating temperature of a refrigerant in the evaporator 12. This action results in the pressure of the refrigerant supplied from a condenser 18 is being reduced to make it easy for the refrigerant to evaporate in the evaporator 12. This action results in the flow rate of the refrigerant flowing into the inlet of the evaporator 12 being under a negative feed-back controlled to make the superheat reach a certain value at which the ability of the evaporator 12 is maximum (or the refrigerating ability of the refrigeration system is maximum). The reduction of the superheat to zero or less means that all of the refrigerant is not changed from liquid to vapor in the evaporator 12 so that the liquid refrigerant flows into a compressor 20 and breaks or damages the compressor 20. Therefore, the certain value of the superheat is over zero, but however, if the certain value is too large the refrigeration reduces its refrigerating ability in comparison with its input.
In FIG. 2, a power element 22 controls the valve opening of the valve unit 14 to make the changing practical superheat equal to the certain value. Power element 22 has a diagram 26 connected to the valve body 24 through an actuating rod, and produces force opposing an urging force of an urging means 28 for urging valve body 24 to its closed position. As shown in FIG. 1, superheated vapor of the refrigerant is introduced from the outlet of evaporator 12 to one side of the diaphragm 26 through a pipe 30, while a base end of pipe 32 whose extended end is connected to thermo bulb 16 is located on the other side of the diaphragm 26.
Valve unit 14 having the above-constructed power element 22 is well known.
FIG. 3 is a vertically cross-sectioned view showing the conventional temperature sensor 16 for sensing the temperature of superheated refrigerant vapor. Thermo bulb 16 has a bulb cylinder 36 which is communicated with an extended end of the pipe 32 at one end thereof and which is sealed by sealing a plug 34 at the other end thereof. Bulb cylinder 36 is made of material such as copper having a high thermal conductivity. A rod-like thermal ballast 38 having a rectangular section is housed in the bulb cylinder 36 with its two the supported by metal net-like support members 40. Thermal ballast 38 is made of inorganic material such as asbestos or heat insulating board having a heat capacity higher than a certain value. An operating fluid having the substantially same vapor-liquid equilibrium temperature characteristic as the refrigerant in the refrigeration system is sealed in the bulb cylinder 36 together with a gas such as helium or nitrogen which is not condensed to liquid at a temperature under which the thermostatic expansion valve 10 is used. The gas such as helium or nitrogen applies a certain pre-pressure to that side of the diaphragm 26 on which the base end of the pipe 32 is located.
When an outer thermal-load reduces rapidly in the above constructed refrigeration system and the thermo bulb 16 senses a lowering of the temperature of superheated refrigerant vapor at the outlet of the evaporator 12 to make superheat become less than the above-mentioned certain value, a part of the operating liquid sealed in the bulb cylinder 36 of the thermo bulb 16 located at the outlet of the evaporator 12 changes from gas to liquid on the surface of the bulb cylinder 36 and the time constant of this change is comparatively small. As a result, pressure in the bulb cylinder 36 decreases comparatively quickly and the diaphragm 26 in FIG. 2 moves comparatively quickly upward to make the valve opening smaller. At this time, the flow rate of the refrigerant flowing into the inlet of the evaporator 12 is reduced comparatively quickly to prevent a liquid flood of the refrigerant to the compressor 20.
It is well known that when the liquid refrigerant reaches compressor 20 it will break or damage the latter.
When a thermal-load to the evaporator 12 increases rapidly, or the outer thermal-load increases rapidly, and after the thermo bulb 16 senses an increasing temperature of superheated refrigerant vapor at the outlet of the evaporator 12 so as to make superheat become more than a certain value, the liquid operating fluid which is trapped on porous surfaces of the thermal ballast 38 in the bulb cylinder 36 of the thermo bulb 16 located at the outlet of the evaporator 12 is gasified as the temperature of the thermal ballast 38 rises. The time constant of this change from liquid to gas is comparatively large because the thermal ballast 38 is made of material having low heat conductivity, as described above. As a result, the pressure in the bulb cylinder 36 rises comparatively slowly and the diaphragm 26 moves comparatively slowly downward in FIG. 2 to make the valve opening larger. At this time, the flow rate of the refrigerant flowing into the inlet of the evaporator 12 increases comparatively slowly.
It is well known that this comparatively slow increase the valve opening at the time when the superheat of the refrigerant rises effectively restrains a repeating of a quick increase and a reduction of the valve opening in a short cycle (the hunting phenomenon). The hunting phenomenon produces a repeating of an excess of and a shortage of the refrigerant supplied to the evaporator 12 in a short cycle and reduces the overall efficiency of the refrigeration system. The excess of the refrigerant supplied to the evaporator 12 in the hunting phenomenon causes the refrigerant to be supplied or flooded as liquid to the compressor 20. This causes the compressor 20 to be damaged.
Pressure of the superheated refrigerant vapor is applied from the outlet of the evaporator 12 to the one side of the diaphragm 26 in a power element 22 through the pipe 30. This pressure has a certain relationship with the evaporating temperature of the refrigerant which relationship is a factor for determining the superheat at the exists of the evaporator 12, so that superheat is the difference between the evaporating temperature obtained from the certain relationship and the temperature of the superheated refrigerant vapor at the exits of the evaporator 12.
Since the force of the urging means 28 can be adjusted by an adjusting means 42, a desired valve opening can be freely set to create a desired superheat of a certain value the evaporator 12.
The following problems are caused in the above-described prior art thermo bulb 16.
(1) Asbestos of which thermal ballast 38 is made is a well-known carcinogenic substance that causes lung cancer.
(2) The main raw material of the heat insulating board from which the thermal ballast 38 is made is a natural product including diatomaceous earth and the like. It is therefore impossible to precisely control the components of this raw material unchanged. In addition, this raw material includes a very small amount of various impurities. The characteristic of the thermal ballast 38 thus changes slightly with every batch of products manufactured.
(3). Material such as asbestos and heat insulating board has a gas adsorption characteristic that influences the performance of the thermo bulb 16. The gas adsorption characteristic changes depending upon the temperature and becomes larger as the temperature falls. Therefore, since a thermal-pressure equilibrium characteristic of the fluid sealed in the bulb cylinder 36 is determined in a case in which the thermal ballast 38 is omitted and is remarkably changed, a precise value of the maximum operating pressure of the thermo bulb 16 can not be determined.
(4) Thermal ballast 38 having no gas adsorption characteristic can be made by a metal sinter or foam. However, it is likely to change chemically. Therefore, it is also likely to be chemically altered by any heat produced, such as by brazing, while the thermo bulb 16 and the valve body 14 are being assembled in the manufacturing process the operating fluid sealed in the bulb cylinder 36 is likely to chemically react under catalysis of the thermal ballast 38. Therefore, the thermal ballast 38 made of a metal sinter or foam cannot be used practically unless a specific treatment for making it chemically stable is carried out first. It is often insufficient to employ only the common high temperature vacuum drying process for eliminating from the thermal ballast 38 those gases which have been absorbed in the thermal ballast 38 having a high gas adsorption characteristic in the manufacturing process before the operating liquid is sealed in the bulb cylinder 36.
The above-mentioned specific treatment increases greatly the cost of manufacturing the thermo bulb 16. In addition, the thermal ballast 38 in which gases (except from the operating liquid) are adsorbed makes the performance of the thermo bulb 16 insufficient.